Thermal Storage System

ABSTRACT

An apparatus for storing and retrieving thermal energy from a phase change material including a plurality of heat exchangers. Each one of the plurality of heat exchangers provides the means for transferring energy between the phase change material on a primary side of the heat exchanger and a fluid on a secondary side of the heat exchanger. The phase change material is a mixture of two or more inorganic salts.

TECHNICAL FIELD

The instant invention relates to a thermal storage system. More specifically, the disclosure pertains to energy storage and retrieval from a phase change material.

BACKGROUND

Various types of thermal energy storage systems are well known in the art. In some systems, energy is stored and/or retrieved as sensible heat in either a solid or a liquid, or in a phase change material such as salt, or in a thermally stratified composition of a solid and a heat transfer fluid.

The storage and retrieval of sensible heat is typically accomplished by inducing a temperature difference between a hot and a cold source. As such, sensible energy storage systems are prone to large volume requirements of the liquid or solid storage media and are therefore expensive.

The transfer of energy to and from a phase change material is typically at a constant temperature and in the form of the latent heat of fusion of the material. Because the latent heat of fusion of a material is typically greater than the specific heat capacity of the same material used in a sensible storage system, the amount of the phase change material required for storing an equivalent amount of energy is typically less than the same material required for storing sensible energy. Accordingly, a phase change storage system is less costly than an equivalent sensible energy storage system and, because of their reduced size, the stand-by loses from a phase change storage system will also be less than those for a sensible heat storage system. However, retrieving the energy stored in a phase change material can be problematic in that the phase change material solidifies onto the heat exchanger surfaces when the heat of fusion is extracted from the liquid state. This build-up of the solid acts as an insulating layer, thereby reducing the heat transfer between the phase change material and the energy transport fluid.

Accordingly, there exists a need for an efficient and cost effective energy storage system comprising a phase change material.

SUMMARY

In accordance with an embodiment of the invention, a thermal energy storage system comprises a storage tank housing a phase change material. Thermal energy is stored or retrieved from the phase change material, respectively, by adding or removing energy in the form of the latent heat of fusion of the phase change material.

In an embodiment of the invention, the storage system comprises a first and a second heat exchanger fluidly connected in series such that energy from the phase change material flowing through a primary side of the first and the second heat exchangers is transferred to a first heat transfer fluid flowing through a secondary side of the first and the second heat exchangers. The storage system further comprises a third and a fourth heat exchanger fluidly connected in series such that energy from a second heat transfer fluid flowing through a secondary side of the third and the fourth heat exchangers is transferred to the phase change material on a primary side of the third and the fourth heat exchangers.

In an embodiment of the invention, the phase change material is a dilute eutectic composition comprising a mixture of two or more inorganic salts in a thermodynamic equilibrium state of solid and liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a thermal energy storage system in accordance with an embodiment of the invention; and

FIG. 2 is a thermodynamic phase change diagram for a phase change material in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

While the present invention is subject to various modifications, embodiments illustrating the best mode contemplated by the inventors for carrying out the invention are described in detail herein below by way of examples with reference to the included drawings. However, it should be clearly understood that there is no intention to limit the invention in any form or manner to the disclosed embodiments, forms, or examples. As such, all alternatives are considered as falling within the scope, spirit and intent of the invention as defined by the appended claims.

FIG. 1 is a schematic of thermal energy storage system 10 in accordance with an embodiment of the invention. Thermal storage system 10 includes phase change material 12 contained within tank 14, first heat exchanger 16 fluidly connected in series to second heat exchanger 18 and third heat exchanger 20 fluidly connected in series to fourth heat exchanger 22.

In an embodiment of the invention, each one of the first, second and third heat exchangers 16, 18 and 20, respectively, comprise a primary side and a secondary side. As illustrated in FIG. 1, first heat exchanger 16 comprises primary side 24 and secondary side 26. Similarly, second heat exchanger 18 comprises primary side 28 and secondary side 30; and third heat exchanger 20 comprises primary side 32 and secondary side 34. Each one of first, second and third heat exchangers, 16, 18 and 20, respectively, are one of the various types of heat exchangers as are well known in the art, including a shell-and-tube heat exchanger. In the embodiment of thermal storage system 10, fourth heat exchanger 22 is illustrated in FIG. 1 as comprising plurality of tubes 36 embedded in phase change material 12 within tank 14. As such, plurality of tubes 36 include multiple tubes in a parallel arrangement connected by headers. Alternatively, plurality of tubes 36 form one or more coil of continuous tubing.

A portion of phase change material 12 extracted from tank 14 flows along path 38 and enters primary side 28 of second heat exchanger 18. The phase change material flowing through primary side 28 exits second heat exchanger 18 along path 40 and enters primary side 24 of first heat exchanger 16. The phase change material flowing through primary side 24 exits first heat exchanger 16 along path 42 and enters tank 14. A first fluid flows along path 44 and enters secondary side 26 of first heat exchanger 16. The first fluid flowing through secondary side 26 exits first heat exchanger 16 along path 46 and enters secondary side 30 of second heat exchanger 18. The first fluid flowing through secondary side 30 exits second heat exchanger 18 along path 48.

A second fluid flows along path 50 and enters secondary side 34 of third heat exchanger 20. The second fluid flowing through secondary side 34 exits third heat exchanger 20 along path 52 and enters secondary side 36 of fourth heat exchanger 22. The second fluid flowing through secondary side 36 exits fourth heat exchanger 22 along path 54. A portion of phase change material 12 extracted from tank 14 flows along flow path 56 and enters primary side 32 of third heat exchanger 20. The phase change material flowing through primary side 32 exits third heat exchanger 20 along flow path 58 and enters tank 14.

As is well known to those skilled in the art, each of the first and second heat exchangers 16 and 18, respectively, provide the means for transferring energy between the phase change material and the first fluid, respectively, flowing through the primary and the secondary sides of these heat exchangers. Similarly, third heat exchanger 20 provides the means for transferring energy between the phase change material and the second fluid, respectively, flowing through the primary and the secondary sides of this heat exchanger. Also, fourth heat exchanger 22 provides the means for transferring energy between the second fluid flowing through secondary side 36 and phase change material 12 encasing secondary side 36 of fourth heat exchanger 22.

Whether energy is transferred to the phase change material or from the phase change material flowing through first, second and third heat exchangers 16, 18 and 20, respectively, is determined by the temperatures and the thermodynamic states of the fluid streams entering each primary sides 24, 28 and 32 and secondary sides 26, 30 and 34 of first, second and third heat exchangers 16, 18 and 20, respectively. Similarly, the temperature and the thermodynamic state of the second fluid entering secondary side 36 of fourth heat exchanger 22 will determine whether energy is transferred to or from phase change material 12 encasing secondary side 36.

In accordance with an embodiment of the invention, the first fluid entering secondary side 26 of first heat exchanger 16 is feed water. Within first heat exchanger 16, energy from the phase change material flowing through primary side 24 is transferred to the fluid flowing through secondary side 26, converting the feed water into saturated steam. The saturated steam exits secondary side 26 along flow path 46 and enters secondary side 30 of second heat exchanger 18. Within second heat exchanger 18, energy from the phase change material flowing through primary side 28 is transferred to the fluid flowing through secondary side 30, converting the saturated steam into superheated steam. The superheated steam exits secondary side 30 along flow path 48. In an embodiment of the invention, the superheated steam flowing along flow path 48 is used for operating a steam turbine connected to a generator for generating electricity. In another embodiment, the superheated steam is used for operating a cooling apparatus such as an absorption chiller. In an alternate embodiment, the superheated steam is used for heating an enclosure. In yet another embodiment, the superheated steam is used in an industrial process. As can be seen, the superheated steam can be used in various applications.

As will be apparent to one skilled in the art, the phase change material flowing through primary sides 24 and 28 of first and second heat exchangers 16 and 18, respectively, will undergo a change in its thermodynamic state as energy from the phase change material is transferred to the fluid flowing through secondary sides 26 and 30 of first and second heat exchangers 16 and 18, respectively. In an embodiment of the invention, the phase change material extracted from tank 14 along path 38 is liquid. In another embodiment, the phase change material extracted from tank 14 along path 38 is a slurry comprising liquid and solid in a thermodynamic equilibrium state. Similarly, the phase change material exiting primary side 28 of second heat exchanger 18 and entering primary side 24 of first heat exchanger 16 along flow path 40 is liquid or a slurry comprising liquid and solid in a thermodynamic equilibrium state. In an embodiment of the invention wherein the phase change material entering primary side 28 of second heat exchanger 18 is a slurry, then the fraction of solid in the slurry exiting primary side 28 and entering primary side 24 of first heat exchanger 16 will be relatively more than the fraction of solid in the slurry entering primary side 28. Similarly, if the phase change material entering primary side 24 of first heat exchanger 16 is a slurry, then the fraction of solid in the slurry exiting primary side 24 and returning to tank 14 along path 42 will be relatively more than the fraction of solid in the slurry entering primary side 24.

In accordance with an embodiment of the invention, the second fluid entering secondary side 34 of third heat exchanger 20 is a liquid such as water or oil at a relatively high temperature. In another embodiment, the second fluid entering secondary side 34 is a gas such as saturated or superheated steam. In an alternate embodiment, the second fluid is from a heat source such as a concentrating solar collector or a solar power tower collector.

In an embodiment of the invention, a portion of phase change material 12 extracted from tank 14 flows along path 56 and enters primary side 32 of third heat exchanger 20. The phase change material exiting third heat exchanger 20 is returned to tank 14 along flow path 58. Within third heat exchanger 20, energy from the second fluid flowing through secondary side 34 is transferred to the phase change material flowing through primary side 32. In accordance with an embodiment of the invention, the phase change material enters primary side 32 as a slurry comprising liquid and solid in a thermodynamic equilibrium state and exits as a liquid. In another embodiment of the invention, the phase change material enters and exits primary side 32 as a slurry comprising liquid and solid in a thermodynamic equilibrium state. As such, the fraction of solid in the slurry exiting primary side 32 will be less than the fraction of solid in the slurry entering primary side 32.

As previously stated, fourth heat exchanger 22 is illustrated in FIG. 1 as comprising plurality of tubes 36 embedded in phase change material 12 within tank 14. In accordance with an embodiment of the invention, the phase change material surrounding plurality of tubes 36 is converted from solid to a slurry comprising liquid and solid in a thermodynamic equilibrium state. In another embodiment of the invention, the fraction of solid in the slurry surrounding plurality of tubes 36 is reduced. In an alternate embodiment of the invention, the slurry surrounding plurality of tubes 36 is converted into liquid.

In an embodiment of the invention, the temperature of the phase change material returned to tank 14 along path 58 is relatively higher than the temperature of the phase change material surrounding plurality of tubes 36. Similarly, the phase change material returned to tank 14 along path 42 will be relatively cooler than the phase change material extracted from tank 14 along path 38. As such, phase change material 12 within tank 14 will be thermally stratified. As will be apparent to one skilled in the art, the relatively warmer phase change material along path 58 does not necessarily have to be returned near the top of tank 14 for initiating and/or maintaining tank 14 in a thermally stratified state. Similarly, the relatively cooler phase change material along path 42 does not necessarily have to be returned near the bottom of tank 14 for initiating and/or maintaining tank 14 in a thermally stratified state. In a thermally stratified tank, the phase change material extracted along path 38 will be from near the top of tank 14

In accordance with an embodiment of the invention, phase change material 12 is a mixture of two or more inorganic salts, each of which inorganic salt undergoes a thermodynamic phase change between its respective solid and liquid states. As is well known to those skilled in the art, the temperature at which one or more of the inorganic salts undergo thermodynamic phase change between its respective solid and liquid states is dictated by whether the mixture of the salts forms a eutectic composition or a dilute eutectic composition. Whether the mixture of the two or more inorganic salts forms a eutectic composition or a dilute eutectic composition is defined by one of the weight, volumetric or mole content of each of the two or more inorganic salts.

FIG. 2 shows simple phase change diagram 100 for phase change material 12 comprising a binary mixture of two inorganic salts sodium hydroxide (NaOH) and sodium nitrate (NaNO₃) in accordance with an embodiment of the invention. In an alternate embodiment, the phase change material comprises a mixture of three, or more, inorganic salts. For the binary mixture comprising inorganic salts NaOH and NaNO₃ in an embodiment of the invention, temperature 102 is a function of the weight percent of NaNO₃ 104. As illustrated, the binary mixture comprising about 83.2 percent-by-weight of NaNO₃ and about 16.8 percent-by-weight of NaOH (numeral 106) forms eutectic composition 108 having eutectic temperature of about 246 degrees-C. As can be seen, for the binary mixture comprising more than about 83.2 percent-by-weight of NaNO₃, solidus state 110 is maintained at the eutectic temperature of about 246 degrees-C. Accordingly, the NaOH will always be in the liquid state and, therefore, the binary mixture will be in the form of a slurry comprising liquid NaOH in thermodynamic equilibrium with some NaNO₃ in solid state. In accordance with an embodiment of the invention, phase change material 12 comprises a dilute eutectic composition in NaNO₃ including about 99.0 percent-by-weight of NaNO₃ and about 1.0 percent-by-weight of NaOH (numeral 112).

As is well know to those skilled in the art, salt is a corrosive compound. In an embodiment of the invention, each component of thermal energy storage system 10 is manufactured from material naturally resistant to corrosion from salt. In another embodiment, each component of thermal energy storage system 10 is treated for preventing or minimizing corrosion from salt. In an alternate embodiment, each component of thermal energy storage system 10 is coated with a salt phobic compound for preventing or minimizing corrosion.

Various modifications and additions may be made to the exemplary embodiments presented hereinabove without departing from the scope and intent of the present invention. For example, while the disclosed embodiments refer to particular features, the scope of the instant invention is considered to also include embodiments having different combinations of features different from and/or in addition to those described herein. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, and variations as falling within the scope and intent of the appended claims, including all equivalents thereof. 

1. An apparatus for storing and retrieving thermal energy, the apparatus comprising: a tank containing a phase change material; a first heat exchanger fluidly connected in series with a second heat exchanger, wherein the phase change material from the tank enters a primary side of the second heat exchanger; the phase change material flows through and exits the second heat exchanger and enters a primary side of the first heat exchanger; the phase change material flows through and exits the first heat exchanger and enters the tank; a first fluid enters and flows through a secondary side of the first heat exchanger wherein energy is transferred between the phase change material and the first fluid; and the first fluid exits the first heat exchanger and enters and flows through a secondary side of the second heat exchanger wherein energy is transferred between the phase change material and the first fluid; and a third heat exchanger fluidly connected in series with a fourth heat exchanger, wherein the fourth heat exchanger is embedded within the phase change material within the tank; a second fluid from an energy source enters and flows through a secondary side of the third heat exchanger; the second fluid exits the third heat exchanger and enters and flows through a secondary side of the fourth heat exchanger wherein energy is transferred between the second fluid and the phase change material surrounding the secondary side of the fourth heat exchanger; the second fluid exits the fourth heat exchanger and returns to the energy source; the phase change material from the tank enters and flows through a primary side of the third heat exchanger wherein energy is transferred between the phase change material and the second fluid; and the phase change material exits the third heat exchanger and enters the tank.
 2. The apparatus of claim 1, wherein the phase change material comprises a composition in a thermodynamic equilibrium state of liquid and solid.
 3. The apparatus of claim 1, wherein each of the first, the second and the third heat exchangers comprise shell and tube heat exchangers; the primary side of each of the first, the second and the third heat exchangers comprises a shell; and the secondary side of each of the first, the second and the third heat exchangers comprises a plurality of tubes.
 4. The apparatus of claim 1, wherein the first fluid entering the first heat exchanger includes feed water; the first fluid exiting the first heat exchanger and entering the second heat exchanger comprises saturated steam; and the first fluid exiting the second heat exchanger comprises superheated steam.
 5. The apparatus of claim 4, wherein the superheated steam is used for generating electricity.
 6. The apparatus of claim 1, wherein the secondary side of the fourth heat exchanger comprises a plurality of tubes embedded within the phase change material; and the second fluid flows through the plurality of tubes.
 7. The apparatus of claim 6, wherein the phase change material comprises a mixture of at least two or more inorganic salts.
 8. The apparatus of claim 7, wherein the mixture comprises 83.2 percent by weight sodium nitrate and 16.8 percent by weight sodium hydroxide.
 9. The apparatus of claim 1, wherein the phase change material comprises a dilute eutectic composition.
 10. The apparatus of claim 9, wherein the dilute eutectic composition comprises a mixture of at least two or more inorganic salts.
 11. The apparatus of claim 10, wherein the dilute eutectic composition comprises a dilute eutectic composition in sodium nitrate including sodium nitrate in the range of 98 to 99 percent by weight and sodium hydroxide in the corresponding range of 2 to 1 percent by weight.
 12. The apparatus of claim 10, wherein the inside surface of the shell for each one of the first, the second and the third heat exchanger comprises a coating of a salt phobic compound; and the outside surfaces of the plurality of tubes in each one of the first, the second, the third and the fourth heat exchanger comprise a coating of the salt phobic compound.
 13. The apparatus of claim 1, wherein the phase change material within the tank is thermally stratified.
 14. The apparatus of claim 13, wherein the phase change material entering the second heat exchanger comprises the phase change material extracted from near a top of the tank; and the phase change material exiting the first heat exchanger enters the tank near a bottom of the tank.
 15. The apparatus of claim 14, wherein the phase change material entering the second heat exchanger comprises a liquid state of the phase change material; and the phase change material entering the tank near the bottom of the tank comprises thermodynamic equilibrium state of liquid and solid.
 16. The apparatus of claim 13, wherein the phase change material entering the third heat exchanger comprises the phase change material extracted from near the bottom of the tank; and the phase change material exiting the third heat exchanger enters the tank near the top of the tank.
 17. The apparatus of claim 16, wherein the phase change material entering the third heat exchanger comprises thermodynamic equilibrium state of liquid and solid; and the phase change material exiting the third heat exchanger comprises the phase change material in the liquid state.
 18. The apparatus of claim 13, wherein the phase change material entering the third heat exchanger comprises the phase change material in the liquid state; and the phase change material exiting the third heat exchanger comprises the phase change material in a saturated state.
 19. The apparatus of claim 18, wherein the saturated state is a thermodynamic equilibrium state of liquid and vapor.
 20. The apparatus of claim 18, wherein the saturated state is vapor.
 21. The apparatus of claim 13, wherein the phase change material entering the third heat exchanger comprises the phase change material in the liquid state; and the phase change material exiting the third heat exchanger comprises the phase change material in a superheated vapor state.
 22. The apparatus of claim 13, wherein the fourth heat exchanger is positioned near the bottom of the tank.
 23. The apparatus of claim 22, wherein the phase change material near the bottom of the tank comprises thermodynamic equilibrium state of liquid and solid.
 24. The apparatus of claim 1, wherein the energy source includes a heating source.
 25. The apparatus of claim 1, wherein the energy source includes a cooling source.
 26. A method for storing and retrieving thermal energy from a phase change material within a tank, the method comprising the steps of: transferring energy from the phase change material to a first fluid; and transferring energy from a second fluid to the phase change material.
 27. The method of claim 26, further comprising the steps of: flowing the first fluid through a secondary side of a first heat exchanger; flowing the first fluid exiting the first heat exchanger through a secondary side of the second heat exchanger; extracting the phase change material from the tank; flowing the phase change material through a primary side of the second exchanger; transferring energy from the phase change material flowing through the second heat exchanger to the first fluid flowing through the second heat exchanger; flowing the phase change material exiting the second heat exchanger through a primary side of the first heat exchanger; transferring energy from the phase change material flowing through the first heat exchanger to the first fluid flowing through the first heat exchanger; and returning the phase change material exiting the first heat exchanger to the tank.
 28. The method of claim 27, further comprising the steps of: extracting the phase change material from the tank; flowing the phase change material through a primary side of a third heat exchanger; returning the phase change material from the third heat exchanger to the tank; extracting the second fluid from an energy source and flowing the second fluid through a secondary side of the third heat exchanger; transferring energy from the second fluid flowing through the third heat exchanger to the phase change material flowing through the third heat exchanger; flowing the second fluid exiting the third heat exchanger through a secondary side of a fourth heat exchanger, wherein the secondary side of the fourth heat exchanger is embedded within the phase change material in the tank; transferring energy from the second fluid flowing through the fourth heat exchanger to the phase change material surrounding the secondary side of the fourth heat exchanger; and returning the second fluid exiting the fourth heat exchanger to the energy source.
 29. The method of claim 28, further comprising the step of maintaining the phase change material in a thermodynamic equilibrium state of liquid and solid.
 30. The method of claim 29, further comprising the step of maintaining the phase change material in a thermally stratified state.
 31. The method of claim 30, wherein the step of flowing the phase change material through the second heat exchanger further comprises the step of extracting the phase change material from near a top of the tank; and the step of returning the phase change material exiting the first heat exchanger to the tank further comprises the step of returning the phase change material near a bottom of the tank.
 32. The method of claim 31, wherein the step of flowing the phase change material through the third heat exchanger further comprises the step of extracting the phase change material from near the bottom of the tank; and the step of returning the phase change material exiting the third heat exchanger to the tank further comprises the step of returning the phase change material near the top of the tank. 